LAUGHTON
ELECTRONICS

Foiled Again: a
stepper and PLC deliver film media

In this project, a system
I
provided was merged with an already- existing design. The customer is a
manufacturer of high quality hot foil stamping presses, and they were
developing a new machine to add to their product line.

Hot foil stamping is used
to add visual appeal to familiar items such as credit cards, retail
boxes and Award Ribbons. It works by
pressing an embossed, heated die against a pigmented
foil and onto the target medium. The result is a
shiny, metallic image unattainable by using ink.

The foil is supplied in
a continuous
ribbon which needs to be advanced following each impression. My job was
to set
up a stepper motor system for advancing the foil. A
key goal with this project was that the
"pull," or draw length, would be set electronically.
Compared to
the crank- driven foil advance used on other machines, this would be
much easier for the
machine operator to adjust.

Another innovation was a
photocell, positioned to observe registration marks on the
foil. That's useful for jobs in which the foil itself bears pre-printed
features
— a repeated image — and it's necessary that every
pull of
the foil should bring one new image exactly into place. The
photocell acts to halt the pull when it reads the registration mark,
which is included as part of the image.
Without registration such jobs would be impossible. Thus the
new foil advance system endowed the machine with two compelling
advantages.

The components I selected
were off-the-shelf items from a U.S.
automation supply company. These included a miniature
Programmable Logic Controller (PLC), a stepper motor drive board, a
power supply and the actual stepper motor itself. Most of the
wiring was quite straightforward, tying together the stepper drive, the
photocell, a mode switch, and a 24 volt Start pulse originating
elsewhere in the machine. But one aspect presented a novelty.

Clearly there had to
be some way for the machine operator change the draw length, but the
obvious solution — a numeric keypad — was rejected.
My client liked the look of Omron controls, used elsewhere on the
machine, and asked if I could
somehow arrange for an Omron counter to determine the draw length. It
was a desirable idea, since Omron offers compact, slick-looking
counters
with an LED display and
built-in pushbutton thumbwheels
for adjusting the Preset Count. But implementing the idea was a
problem.
The stepper pulses originate from a ramp generator in the PLC but are
actually counted externally by the counter, which only has a single
output. That implies that the PLC won't have the advance warning it
needs in order to decelerate the motor speed prior
to the terminal count and photocell signals. The solution to this
seemingly intractable problem required an original approach which I
won't document here.

Supplied with the
schematic, parts list and PLC program files, the customer proceeded to
incorporate my design into regular production at their factory. Indeed,
I was pleased to learn later that they also successfully adapted the
system for use in some custom, special order machines.